The present invention relates to a compact ultra high efficiency gas fired steam generator. The apparatus produces low pressure or atmospheric steam for use in humidification of air and various heating applications in heating, ventilation and air conditioning of buildings and in various industrial processes requiring clean steam.
To carry out many industrial processes and manufacturing operations it is often necessary to maintain the temperature and humidity of the working environment and the space air at specific preset temperature-humidity conditions.
In the forced air heating, ventilation and air conditioning of industrial, commercial, institutional and residential buildings the required humidity of the air is most conveniently maintained by addition of the atmospheric steam into the building""s recirculating air. For such humidification of the air the generation of the low pressure or the atmospheric steam on site is the most economic and often the only available alternative.
Technical and commercial literature indicate, that the current art compact isothermic humidifiers are producing the atmospheric steam by boiling and evaporating the incoming feed water at atmospheric pressure in sealed water tanks. The required heat is provided either by electric power via two or more electrodes or resistance heating elements submerged in the boiling water, or by a pressure steam delivered from a central steam boiler to a submerged heat exchanger, or by combustion of natural gas in immersion tube burners in water tanks.
Disclosed in U.S. Pat. No. 5,816,496 is a gas fired steam generator-humidifier with an integrated combustion chamber-heat exchanger submerged in a rectangular water tank.
Disclosed in my earlier applications Ser. No. 08/657,179 entitled Compact Gas Fired Steam Generator, filed Jun. 3, 1996 and Ser. No. 09/419,577 entitled Compact Gas Fired Humidifier, filed Oct. 18, 1999 is a compact gas fired steam generator comprising an insulated dry firebox combustion chamber integrated with a vertical finned tube boiler with a high rate internal reticulation of boiling water.
In the current art gas fired steam generator-humidifiers operating with feed water containing disolved solids (DS) the flow of the feed water into the water tank is periodic and the steam generation process is cyclic. The flow of the feed water into the water tank is maintained by solenoid valves and water level switches with the water level in the water tank fluctuating between a high and low water level limits. The cyclic steam generation process includes two operating periods. The first operating period involves steps including transfer of heat from the heat source into the boiling water, evaporation of the boiling water, precipitation of DS and concentration of total solids (TS) in the boiling water, separation of the produced steam from the boiling water, and discharge of the produced steam from the water tank. The following second operating period, in addition to the above steps, includes filling up of the water tank with make up water, draining of a small portion of the boiling water, and heating the feed water to its boiling temperature.
Because of the periodic intake of cool feed water into the water tank, the rate of production of steam during the second operating period is reduced. Depending on the flow rate of feed water, temperature of the water in the water tank may fall below the boiling point when the steam is not produced. As a result, the steam is produced at an unsteady rate and the overall thermal efficiency is limited and at the 80% level. It is therefore desirable to provide an improved steam generator maintaining continuous steady state production of steam at higher thermal efficiency.
The current art gas fired steam generator-humidifiers also have a water seal to allow for periodic overflow of the blow down boiling water while preventing steam from escaping through the overflow outlet. With such overflow water seal arrangement, the steam pressure in the tank is limited by the height of the overflow tube. It is therefore also desirable to provide an improved steam generator that would operate at higher steam pressures.
The feed water used in current art steam generator-humidifier systems may be a city water, softened water, deionized water (DI) or reverse osmosis treated water (RO). As the feed water is converted to steam, impurities which enter with feed water are concentrated and the inorganic hard scale forming substances such as calcium and magnesium precipitate. The precipitated substances build up as hard scale on the submerged heat transfer surfaces requiring regular cleaning of the water tank. It is therefore also desirable to provide an improved steam generator that would provide minimum build up of the hard scale on the heat transfer surfaces.
To provide the required heat transfer area, the current art gas fired water tank steam generator-humidifiers require relatively large and heavy water tanks, containing large volume of boiling water causing a delayed response in steam production. It is therefore also desirable to provide an improved steam generator with minimum volume of the boiling water permitting an instantaneous production of steam.
Briefly, and in general terms, the present invention provides for a compact gas fired steam generator producing a steady state continuous flow of clean low pressure or atmospheric steam at an ultra high thermal efficiency for use in humidification of air and in various heating applications in heating, ventilation and air conditioning of buildings and in industrial processes requiring clean steam.
The improved compact steam generator of the present invention comprises a casing, an insulated firebox chamber integrated with an immersed vertical fin tube boiler including a steam separation chamber, a condensing heat exchanger for recovery of the waste heat from flue gases to preheat the feed water and the return condensate and a heat exchanger to recover waste heat from the blow down boiling water to the incoming feed water. It uses a high rate steam generating process occurring in vertical fin tubes in an internally recirculating boiling water. The flow of feed water and the production of low pressure steam are continuous, with the produced steam being separated from the recirculating boiling water in the boiler""s steam separation chamber. The water level in the steam separation chamber is maintained at a constant water level by a float switch operating a solenoid valve controlling the continuous overflow of a small portion of the recirculating boiling water to drain to maintain the concentration of dissolved and suspended solids in the internally recirculating boiling water at a preset level. The high rate internal reticulation of boiling water, the extended heat transfer surfaces of the vertical finned tubes and the effective contact of the combustion gases with the extended heat transfer surfaces provide an exceptionally compact apparatus offering ultrahigh thermal efficiency and substantionally maintenance free unattended operation.
In another preferred embodiment of the present invention the steam is produced at an atmospheric pressure and the water level in the steam separation chamber is maintained at a constant water level by a continuous overflow of a small portion of the recirculating boiling water through a water seal to drain to maintain the concentration of dissolved and suspended solids in the internally recirculating boiling water at a preset level while preventing steam from escaping through the overflow outlet.
Another aspect of the present invention is operation of the compact steam generator at a negative pressure to prevent leakage of combustion products into the building and to permit addition of dilution air to the cooled combustion products to lower their dew point to eliminate condensation of moisture in flue duct.
These and other features and advantages of this invention will become apparent from the following detailed description, and the accompanying drawings, which illustrate by way of example the features of the invention.